Solid electrolyte laminate, method for manufacturing solid electrolyte laminate, and fuel cell

ABSTRACT

Provided is a solid electrolyte laminate comprising a solid electrolyte layer having proton conductivity and a cathode electrode layer laminated on one side of the solid electrolyte layer and made of lanthanum strontium cobalt oxide (LSC). Also provided is a method for manufacturing the solid electrolyte. This solid electrolyte laminate can further comprise an anode electrode layer made of nickel-yttrium doped barium zirconate (Ni—BZY). This solid electrolyte laminate is suitable for a fuel cell operating in an intermediate temperature range less than or equal to 600° C.

TECHNICAL FIELD

The invention of the present application relates to a solid electrolytelaminate, a method for manufacturing a solid electrolyte laminate, and afuel cell. Specifically, the invention relates to a solid electrolytelaminate that can offer high efficiency in a fuel cell operating in anintermediate temperature range less than or equal to 600° C. and thatcan be easily manufactured, a method for manufacturing the solidelectrolyte laminate, and a fuel cell.

BACKGROUND ART

A solid oxide fuel cell (hereinafter referred to as “SOFC”) is highlyefficient and does not require an expensive catalyst such as platinum.On the other hand, since its operating temperature is as high as 800° C.to 1000° C., a problem arises in that a structural material such as aninterconnector is likely to be degraded.

To solve the above-described problem, an intermediate temperatureoperating SOFC having a lowered operating temperature less than or equalto 600° C. has been expected. At low operating temperatures, however,efficiency is decreased, so that predetermined power generationperformance cannot be ensured disadvantageously. Therefore, a solidelectrolyte exhibiting high efficiency even at low operatingtemperatures and being capable of ensuring predetermined powergeneration performance has been required.

As a solid electrolyte, one having oxygen ion conductivity or protonconductivity is employed. In the case of employing a solid electrolytehaving oxygen ion conductivity, the oxygen ion is bonded to hydrogen toproduce water at a fuel electrode. This water dilutes fuel to decreaseefficiency disadvantageously.

On the other hand, a solid electrolyte having proton conductivity suchas yttrium-doped barium zirconate (hereinafter referred to as “BZY”) canachieve high proton conductivity at low temperatures since activationenergy for charge transfer is low, and is expected as a solidelectrolyte material as an alternative to the above-described solidelectrolyte having oxygen ion conductivity. In the case of employing thesolid electrolyte having proton conductivity, the above-describedproblem encountered in the solid electrolyte having oxygen ionconductivity does not occur.

CITATION LIST Patent Document PTD 1: Japanese Patent Laying-Open No.2001-307546 SUMMARY OF INVENTION Technical Problem

For the above-described intermediate temperature operating SOFC, inorder to ensure power generation performance, an electrode material madeof platinum (Pt) having high electrical conductivity, lanthanumstrontium manganese oxide (LSM) or the like is often employed for acathode electrode (air electrode).

The above-described Pt, however, is very expensive. The use of theabove-described LSM may increase an overvoltage, which makes itdifficult to ensure required power generation performance.

On the other hand, lanthanum strontium cobalt oxide (LSC) used as acathode electrode material in an oxygen ion conducting fuel cell of YSZ(yttria-stabilized zirconia) or the like is suitable for use as acathode electrode in the above-described fuel cell because of its higherelectrical conductivity and electrocatalytic activity than those of theabove-described LSM. LSC, however, has small thermodynamic stability ina reducing atmosphere, and depending on the type of a solid electrolytelayer to be laminated, characteristics as a cathode are often degradedby a product of a reaction with the solid electrolyte. Moreover,internal stress is likely to be produced within an LSC film formed by asolid phase method. Therefore, problems arise in that detachment fromelectrodes and cracks are likely to occur, and it is difficult to form acathode electrode layer (air electrode).

On the other hand, the above-described yttrium-doped barium zirconate(BZY) having proton conductivity has excellent chemical stability, butis disadvantageous in that sinterability is poor as a polycrystallinematerial and in that the ratio of grain boundary is large because of itssmall crystal grains, which inhibits proton conductivity and decreasesthe total electric conductivity. Therefore, the above-described BZY hasnot been effectively utilized so far.

In particular, if the doped amount of yttrium is less than or equal to10 mol %, crystal grains are difficult to grow at the time of sintering.Thus, the grain boundary surface density increases to increase theresistance. If this is utilized for a fuel cell, power generationperformance will be decreased.

On the other hand, if more than or equal to 15 mol % of yttrium isdoped, it will be difficult to uniformly dissolve yttrium in a dispersedmanner. Therefore, in the temperature range of 200° C. to 400° C., aphenomenon will occur in which relaxation of a non-equilibrium phaseoccurs to change the coefficient of thermal expansion.

FIG. 4 shows changes in lattice constant of solid electrolytes havingdifferent amounts of yttrium doped, with respect to temperature changes.As shown in this drawing, in the case where yttrium is not doped, therate of change in lattice constant with respect to temperature changesis substantially constant, and the lattice constant increases as alinear function along a linear graph having a predetermined gradient. Onthe other hand, as the doped amount of yttrium is increased, the latticeconstant with respect to an identical temperature increases at a certainratio, and in a temperature range around 400° C., a region appears inwhich the lattice constant greatly increases in value deviating from thevicinity of the straight line of the linear function. Theabove-described region where the lattice constant greatly increasesappears when the doped amount of yttrium exceeds 15 mol %, and becomesremarkable at 20 mol %. This is presumed because relaxation of thenon-equilibrium phase has occurred in the temperature range around 400°C.

It is noted that the above-described lattice constant is calculated bythe Rietveld analysis from a high-temperature XRD measurement result.

Since the lattice constant represents the length of each side of unitlattice of crystal, the coefficient of thermal expansion will change inaccordance with the above-described changes in lattice constant. Thatis, when the doped amount of yttrium is increased, the coefficient ofthermal expansion greatly changes in the region around 400° C. In thecase of using the above-described solid electrolyte for a fuel cell,electrode layers are laminated on the both sides of a thin-film solidelectrolyte layer. The materials constituting the above-describedelectrode layers have substantially constant coefficients of thermalexpansion, and thermally expand in proportion to the temperature.Therefore, in the case of employing a solid electrolyte having a largedoped amount of yttrium, large shearing force is produced around 400° C.at the interface in a laminate formed by laminating the above-describedsolid electrolyte and the above-described electrode materials, raisingproblems in that cracks occur in the solid electrolyte layer and theelectrode layers are detached. As a result, yields in the manufacturingprocess and durability of fuel cell cannot be ensured disadvantageously.

The invention of the present application was devised to solve theabove-described problems, and has an object to provide a protonconductive solid electrolyte laminate having good compatibility with asolid electrolyte layer having proton conductivity and having a cathodeelectrode layer made of inexpensive LSC employed therein, and to providea solid electrolyte laminate that solves the above-described problemsencountered in a solid electrolyte layer formed from yttrium-dopedbarium zirconate containing a large doped amount of yttrium and thatoffers high efficiency at an operating temperature less than or equal to600° C. by combination with the above-described cathode electrode layermade of LSC.

Solution to Problem

The invention defined in claim 1 of the present application relates to asolid electrolyte laminate including a solid electrolyte layer havingproton conductivity, and a cathode electrode layer laminated on one sideof the above-described solid electrolyte layer and made of lanthanumstrontium cobalt oxide (LSC).

The solid electrolyte laminate according to the invention of the presentapplication is a combination of a solid electrolyte layer having protonconductivity and offering high power generation efficiency at anoperating temperature less than or equal to 600° C. and a cathodeelectrode made of inexpensive, high-performance LSC.

The above-described LSC has conventionally been used as a cathodeelectrode of oxygen ion conducting fuel cells in many cases, and has notbeen used for fuel cells including a proton conductive solid electrolytelayer.

Inventors of the invention of the present application explored variouselectrode materials in order to develop cathode electrode materialssuitable for fuel cells including a proton conductive solid electrolyte.As a consequence, they found out that the use of the above-described LSCas a cathode electrode resulted in high power generation performance,thereby yielding the invention of the present application.

Optimum electrode materials for an oxygen ion conducting fuel cell and aproton conductive fuel cell are basically different because differentelectrode reactions occur in these fuel cells. The reason why theabove-described LSC is used for oxygen ion conducting fuel cells isconsidered because oxygen vacancy of these materials results in goodoxygen ion conductivity and electronic conductivity.

Although the factor that the above-described LSC can exert highperformance when used as a cathode electrode of a proton conductive fuelcell has not been clarified, it is considered because surface diffusionof proton in the cathode electrode becomes dominant in a cathodereaction in the proton conductive fuel cell, and the above-described LSCserves favorably as a cathode electrode material.

Preferably, a solid electrolyte formed from yttrium-doped bariumzirconate (hereinafter, BZY) is employed as the above-described protonconductive solid electrolyte. A solid electrolyte layer made of BZYaccording to the invention of the present application is adjusted suchthat the above-described doped amount of yttrium is 15 mol % to 20 mol %(more than or equal to 15 mol % and less than or equal to 20 mol %), andthe rate of increase in lattice constant of the above-described solidelectrolyte at 100° C. to 1000° C. (more than or equal to 100° C. andless than or equal to 1000° C.) with respect to temperature changes issubstantially constant.

In the invention of the present application, the doped amount of yttriumis set at 15 mol % to 20 mol %. Accordingly, high proton conductivitycan be ensured, and sinterability can be improved.

If the above-described doped amount of yttrium is less than 15 mol %,changes in coefficient of thermal expansion will be relatively small, sothat the problem due to thermal expansion will be less likely to occur.However, in order to improve sinterability and ensure protonconductivity in the above-described intermediate temperature range, itis preferable to set the doped amount of yttrium at more than or equalto 15 mol %. On the other hand, if the above-described doped amount ofyttrium exceeds 20 mol %, it will be difficult to uniformly blendyttrium in a dispersed manner.

Furthermore, in the solid electrolyte according to the invention of thepresent application, the rate of increase in lattice constant at 100° C.to 1000° C. with respect to temperature changes is made substantiallyconstant. That is, relaxation of the non-equilibrium phase does notoccur in the above-described temperature range, and the coefficient ofthermal expansion is held substantially constant. Therefore, in the stepof laminating electrode layers and the like, occurrence of cracks thatwould be caused by changes in coefficient of thermal expansion can beprevented, and the electrode layers are unlikely to be detached. Herein,the expression “substantially constant” means that, when plotting thelattice constant with respect to the temperature in the temperaturerange more than or equal to 100° C. and less than or equal to 1000° C.to create a scatter plot, the lattice constant increases as a linearfunction and does not exhibit specific changes around 400° C.

Particularly, in the invention of the present application, LSC isemployed as a cathode electrode material. Although LSC is likely tocause detachment from a solid electrolyte and cracks as described above,such problems will not occur because the coefficient of thermalexpansion is adjusted to be constant in the solid electrolyte layeraccording to the invention of the present application.

The rate of increase in lattice constant of the above-described solidelectrolyte at 100° C. to 1000° C. with respect to temperature changesis preferably set at 3.3×10⁻⁵ Å/° C. to 4.3×10⁻⁵ Å/° C. (more than orequal to 3.3×10⁻⁵ Å/° C. and less than or equal to 4.3×10⁻⁵ Å/° C.). Bysetting the rate of increase in lattice constant at the above-describedrange, it becomes possible to set the coefficient of thermal expansionat a predetermined range. It is more preferable to set theabove-described rate of increase in lattice constant such that theaverage coefficient of thermal expansion at 100° C. to 1000° C. becomes5×10⁻⁶(1/K) to 9.8×10⁻⁶(1/K).

The sintering temperature after molding the solid electrolyte accordingto the invention of the present application as a thin film andlaminating the above-described electrode materials on this thin-filmsolid electrolyte is approximately 1000° C. Therefore, by setting thelattice constant at 100° C. to 1000° C. at the above-described values, agreat difference in thermal expansion amount between the solidelectrolyte layer and the electrode layers will not occur in the step ofsintering the electrode layers, which can effectively prevent cracks anddetachment from occurring.

Preferably, the mean diameter of crystal grains of the above-describedsolid electrolyte is set at more than or equal to 1 μm.

As described above, when the mean diameter of crystal grains of thesolid electrolyte is decreased, the grain boundary surface densityincreases to increase the resistance, and proton conductivity isreduced. By setting the mean diameter of crystal grains at more than orequal to 1 μm, the above-described problems can be avoided. It is notedthat the mean diameter of crystal grains is preferably less than orequal to 30 μm from the viewpoint of film thickness. Herein, the meandiameter of crystal grains refers to an arithmetic mean value of anequivalent diameter of a circle that has the same area as the projectedarea measured for 100 crystal grains in an observation visual field whenmonitoring a surface (or a cross section) of a solid electrolyte as acompact by electron microscope under a magnification of ×1000 to ×5000.

Preferably, the lattice constant of the solid electrolyte at roomtemperature (30° C.) is set at 4.218 Å to 4.223 Å (more than or equal to4.218 Å and less than or equal to 4.223 Å).

The lattice constant at room temperature is correlated with the dopedamount of yttrium and the change in lattice constant around 400° C.Therefore, by setting the lattice constant at room temperature at theabove-described range, the lattice constant around 400° C. can beestimated to grasp the coefficient of thermal expansion of the solidelectrolyte. Moreover, when sintering the laminated electrode layers,detachment and the like can be prevented.

Proton conductivity of the solid electrolyte at 400° C. to 800° C. (morethan or equal to 400° C. and less than or equal to 800° C.) is set at 1mS/cm to 60 mS/cm (more than or equal to 1 mS/cm and less than or equalto 60 mS/cm). Since the above-described proton conductivity can beensured in the above-described temperature range, it becomes possible toensure required power generation performance in an intermediatetemperature range when implementing a fuel cell.

The above-described LSC is employed as a cathode electrode of a solidelectrolyte laminate according to the invention of the presentapplication. On the other hand, it is preferable to provide an anodeelectrode laminated on the other side of the above-described solidelectrolyte layer and made of nickel-yttrium doped barium zirconate(hereinafter, Ni—BZY). Ni has high catalytic activity as a reductioncatalyst. By adding Ni to the above-described BZY, a solid electrolytelaminate that can offer high performance can be formed. It is notedthat, besides the above-described Ni—BZY, Ne—Fe alloy or Pd can beemployed as the cathode electrode.

The above-described solid electrolyte laminate is manufactured by amanufacturing method including the following steps. Specifically, afirst grinding step of mixing and grinding BaCO₃, ZrO₂ and Y₂O₃, a firstheat treatment step of heat treating a mixture (first mixture) havingundergone the above-described grinding at a predetermined temperaturefor a predetermined time, a second grinding step of grinding the mixture(first mixture) having undergone the above-described first heattreatment step again, a first compression molding step of compressionmolding the mixture (second mixture) having undergone theabove-described second grinding step, a second heat treatment step ofheat treating a compact (first compact) having undergone theabove-described compression molding at a predetermined temperature, athird grinding step of grinding the compact (first compact) havingundergone the above-described second heat treatment step, a secondcompression molding step of compression molding a ground product havingundergone the above-described third grinding step, a solid electrolytesintering step of heat treating a compact (second compact) molded by theabove-described second compression molding step at a temperature of1400° C. to 1600° C. (more than or equal to 1400° C. and less than orequal to 1600° C.) for at least 20 hours in an oxygen atmosphere, athird heat treatment step of holding a sintered compact having undergonethe above-described solid electrolyte sintering step at a temperaturelower than in the above-described solid electrolyte sintering step for apredetermined time, a cathode electrode material laminating step oflaminating an electrode material made of lanthanum strontium cobaltoxide (LSC) on one side of the sintered compact having undergone theabove-described third heat treatment step, an anode electrode materiallaminating step of laminating an electrode material made ofnickel-yttrium doped barium zirconate (Ni—BZY) on the other side of thesintered compact having undergone the above-described third heattreatment step, and an electrode material sintering step of heating to atemperature higher than a sintering temperature of the above-describedelectrode materials are included.

The blending amount of above-described BaCO₃, ZrO₂ and Y₂O₃ is notparticularly limited. For example, when the above-described doped amountof yttrium is set at 15 mol %, a material containing 62 wt % of BaCO₃,33 wt % of ZrO₂ and 5 wt % of Y₂O₃ mixed therein can be employed.

In the invention of the present application, a solid electrolytesintered compact is formed by a solid phase reaction method. Thetechnique for carrying out the above-described grinding steps is notparticularly limited. For example, the grinding steps can be carried outby already-known ball milling. For example, as the first grinding stepand the second grinding step, ball milling can be carried out for about24 hours. Although the ground grain size after the above-describedgrinding steps is not particularly limited, grinding is preferablyperformed such that the mean particle diameter is less than or equal to355 μm.

The above-described first heat treatment step can be carried out by, forexample, holding at 1000° C. for about 10 hours in an atmosphere, andthe above-described second heat treatment step can be carried out byholding at 1300° C. for about 10 hours in an atmosphere.

The technique for carrying out the above-described compression moldingsteps is also not particularly limited. For example, a ground materialcan be molded uniaxially to form a predetermined compact. Theabove-described compression molding steps are to uniformly mix therespective blended components in a dispersed manner. As long as grindingcan be performed easily, the form of compact is not particularlylimited. For example, a cylindrical die having a diameter of 20 mm isused, and a compressive force of 10 MPa is applied in the axialdirection, so that a disc-like compact can be formed.

By heat treating the above-described compact at about 1300° C. for about10 hours, the second heat treatment step of dissolving each componentpowder to form a material in which the above-described respectivecomponents have been uniformly dispersed is carried out. Thereafter, thethird grinding step of grinding the compact having undergone theabove-described second heat treatment step is carried out. To uniformlymix the above-described respective component powders in a dispersedmanner, it is desirable to repeatedly carry out the above-describedfirst compression molding step, the second heat treatment step and theabove-described third grinding step in this order. Accordingly, amaterial in which the respective components have been uniformlydissolved in a dispersed manner can be formed. Whether theabove-described respective component powders have been uniformlydispersed can be confirmed with an X-ray diffractometer (XRD).

Next, a second compression molding step of compression molding theground product having undergone the above-described third grinding stepis carried out. The above-described second compression molding step canbe carried out by adding a binder such as ethyl cellulose andcompression molding the above-described ground product. Theabove-described second compression molding step is to mold theabove-described ground product into the form of a solid electrolytelayer, and for example, the product can be molded into a disc havingpredetermined thickness.

Next, by carrying out the sintering step of heat treating theabove-described compact at a temperature of 1400° C. to 1600° C. for atleast 20 hours in an oxygen atmosphere, a required solid electrolytesintered compact can be obtained.

The lattice constant of the solid electrolyte obtained through theabove-described steps exhibits specific changes in the temperature rangearound 400° C. as described above. Therefore, in the invention of thepresent application, the third heat treatment step of holding theabove-described solid electrolyte sintered compact at a temperaturelower than in the above-described sintering step for a predeterminedtime is carried out.

The above-described third heat treatment step is not particularlylimited as long as characteristics which will not cause changes inlattice constant can be imparted to the above-described solidelectrolyte sintered compact. For example, the above-described thirdheat treatment step can be carried out by holding at a temperature of400° C. to 1000° C. (more than or equal to 400° C. and less than orequal to 1000° C.) for 5 hours to 30 hours (more than or equal to 5hours and less than or equal to 30 hours).

By carrying out the above-described third heat treatment step, thelattice constant does not specifically change in the temperature rangearound 400° C., so that the rate of increase in lattice constant at 100°C. to 1000° C. with respect to temperature changes can be madesubstantially constant.

The above-described third heat treatment step can be carried out aftercooling the above-described solid electrolyte sintered compact toordinary temperature after the above-described sintering step.Alternatively, the above-described sintering step and theabove-described third heat treatment step can be carried outsequentially.

The cathode electrode material laminating step of laminating anelectrode material made of lanthanum strontium cobalt oxide (LSC) on oneside of the solid electrolyte sintered compact having undergone theabove-described third heat treatment step is carried out.

The cathode electrode material laminating step can be carried out bydissolving the above-described cathode electrode material in the form ofpowder into a solvent to obtain a paste and applying the paste on theone side of the solid electrolyte sintered compact by screen printing orthe like.

An anode electrode material laminating step of laminating an electrodematerial made of nickel-yttrium doped barium zirconate (Ni—BZY) on theother side of the solid electrolyte sintered compact having undergonethe above-described third heat treatment step is carried out.

The anode electrode material laminating step can be carried out bygrinding and mixing powder made of NiO and BZY with a ball mill and thendissolving it in a solvent to form paste, and applying the paste to theother side of the above-described solid electrolyte sintered compact byscreen printing or the like.

Either of the above-described cathode electrode material laminating stepand the above-described anode electrode material laminating step may becarried out first.

After terminating the above-described cathode electrode materiallaminating step and the above-described anode electrode materiallaminating step, an electrode material sintering step of heating to orabove a sintering temperature of the above-described electrode materialsis carried out.

The above-described electrode material sintering step can be carried outin such a manner as to sinter the both electrodes simultaneously afterterminating the above-described cathode electrode material laminatingstep and the above-described anode electrode material laminating step,or can be carried out separately after each of the above-describedcathode electrode material laminating step and the above-described anodeelectrode material laminating step.

In the above-described manufacturing method, the disc-like sinteredcompact constituting the solid electrolyte layer is first formed, andthe electrode layers are laminated on this disc-like sintered compactserving as a support member, however, the manufacturing method is notlimited to the above-described method.

For example, a technique for first forming a compact constituting theanode electrode, and successively laminating the above-described solidelectrolyte layer and the above-described cathode electrode layer onthis anode electrode compact serving as a support member can beemployed.

The above-described anode electrode compact can be formed by an anodeelectrode material preparing step of mixing Ni with BZY synthesized fromBaCO₃, ZrO₂, and Y₂O₃, and an anode electrode molding step ofcompression molding the above-described anode electrode material to forman anode electrode compact to be the anode electrode layer. Since theabove-described anode electrode serves as a support member, itsthickness is set at approximately 1 mm.

The technique for laminating the solid electrolyte layer on theabove-described anode electrode compact can be achieved as follows. Thatis, similarly to the above-described manufacturing method, theabove-described first grinding step, the above-described first heattreatment step, the above-described second grinding step, theabove-described first compression molding step, the above-describedsecond heat treatment step, and the above-described third grinding stepare carried out to form a ground product of BZY.

Next, a paste forming step of forming the above-described ground productinto paste, and a solid electrolyte laminating step of laminating theabove-described ground product formed into paste on one side of theabove-described anode electrode compact are carried out. Since theabove-described solid electrolyte laminate does not serve as a supportmember, its thickness can be set at 10 μm to 100 μm.

Then, an anode electrode-solid electrolyte sintering step of heattreating the laminate (first laminate) molded in the above-describedsolid electrolyte laminating step at a temperature of 1400° C. to 1600°C. for at least 20 hours in an oxygen atmosphere, and a third heattreatment step of holding the laminate having undergone theabove-described anode electrode-solid electrolyte sintering step at atemperature lower than in the above-described anode electrode-solidelectrolyte sintering step for a predetermined time are carried out.

A cathode electrode material laminating step of laminating a cathodeelectrode material made of lanthanum strontium cobalt oxide (LSC) on oneside of a thin-film solid electrolyte having undergone theabove-described third heat treatment step, and a cathode electrodesintering step of heating to or above a sintering temperature of theabove-described cathode electrode material are carried out. It is notedthat the cathode electrode material laminating step and the cathodeelectrode sintering step can be carried out similarly to theabove-described method. Through these steps, the above-described solidelectrolyte laminate can also be formed.

In the above-described solid electrolyte layer according to theinvention of the present application, the rate of change in latticeconstant with respect to temperature changes is substantially constant,and the coefficient of thermal expansion will not change in accordancewith the temperature. Therefore, cracks in the above-described solidelectrolyte layer or the above-described electrode layers and detachmentof the above-described electrode layers will not occur.

In particular, according to the invention of the present application,cracks in or detachment of the cathode electrode layer will not occurbecause the coefficient of thermal expansion of the above-describedsolid electrolyte layer is adjusted although LSC which is likely tocause cracks and the like is employed as the above-described cathodeelectrode material. Electrode layers having high bondability to thesolid electrolyte layer can thus be provided.

In the invention of the present application, since the above-describedsolid electrolyte has undergone the third heat treatment, the rate ofincrease in lattice constant with respect to temperature changes isconstant in the temperature range of 100° C. to 1000° C., andaccordingly, the coefficient of thermal expansion is also constant. Itis therefore possible to sinter the solid electrolyte layer and theelectrode layers without causing strain and the like. Moreover, sinceinternal stress and the like can be restrained from occurring, a solidelectrolyte laminate having high durability can be formed.

The solid electrolyte according to the invention of the presentapplication is suitable for various types of fuel cells used in atemperature range less than or equal to 600° C., but can also beutilized for fuel cells used in a temperature range more than or equalto 600° C.

Advantageous Effects of Invention

A solid electrolyte laminate that can offer high power generationefficiency in an intermediate temperature range less than or equal to600° C. can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an overall perspective view showing an example of a solidelectrolyte laminate according to the invention of the presentapplication.

FIG. 2 is an enlarged sectional view of an essential part of the solidelectrolyte laminate shown in FIG. 1.

FIG. 3 shows changes in lattice constant of a solid electrolyteaccording to the invention of the present application having undergone aheat treatment and a solid electrolyte not having undergone a heattreatment, with respect to temperature changes.

FIG. 4 shows changes in lattice constant of solid electrolytes havingdifferent amounts of yttrium doped, with respect to the temperature.

FIG. 5 shows an example of a sintering step and a third heat treatmentstep according to the invention of the present application.

FIG. 6 shows another embodiment of the sintering step and the third heattreatment step according to the invention of the present application.

FIG. 7 is a graph showing an example of performance of the solidelectrolyte laminate according to the invention of the presentapplication.

FIG. 8 is an enlarged sectional view showing an essential part ofanother embodiment of the solid electrolyte laminate according to theinvention of the present application.

FIG. 9 is a flow chart showing an example of a manufacturing process ofthe solid electrolyte laminate according to the invention of the presentapplication.

FIG. 10 is a flow chart showing another example of a manufacturingprocess of the solid electrolyte laminate according to the invention ofthe present application.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments according to the invention of the presentapplication will be described based on the drawings.

As shown in FIG. 1, a solid electrolyte laminate 1 constituting a fuelcell is configured to include a solid electrolyte layer 2, an anodeelectrode layer 3 laminated on one side of this solid electrolyte layer2, and a cathode electrode layer 4 formed on the other side.

As solid electrolyte layer 2 according to the present embodiment, asolid electrolyte 2 a made of yttrium-doped barium zirconate(hereinafter, BZY) having proton conductivity is employed.Above-described anode electrode layer 3 is formed by laminating andsintering proton conductive ceramics, and is configured to serve as ananode electrode. On the other hand, above-described cathode electrodelayer 4 according to the present embodiment is formed from lanthanumstrontium cobalt oxide (hereinafter, LSC).

Hereinafter, a method for manufacturing solid electrolyte laminate 1will be described. FIG. 9 shows a flow chart of a manufacturing processof the solid electrolyte laminate.

In the present embodiment, the above-described solid electrolyte isformed by a solid phase reaction method. First, in order to form solidelectrolyte layer 2 made of BZY, 62 wt % of BaCO₃, 31 wt % of ZrO₂ and 7wt % of Y₂O₃ as raw materials are mixed, and a first grinding step iscarried out by ball milling to uniformly mix these raw materials.Thereafter, a first heat treatment step is carried out by heat treatmentat 1000° C. for about 10 hours, and further, a second grinding step iscarried out by performing ball milling on a powder material havingundergone the above-described first heat treatment step. Although thedegree of grinding of the materials in the above-described grindingsteps is not particularly limited, but it is preferable to performgrinding such that the mean particle diameter of ground powder is lessthan or equal to 355 μm.

Next, a compression molding step of uniaxially molding the mixed powderhaving undergone the second grinding step to form a disc-like pressedcompact is carried out. In the above-described compression molding step,for example, a cylindrical die having a diameter of 20 mm is used, and acompressive force of 10 MPa is applied in the axial direction, so that adisc-like compact can be formed.

A second heat treatment step of heat treating the above-describedpressed compact at about 1300° C. for about 10 hours, thereby dissolvingeach component powder to uniformly dissolve each component in adispersed manner is carried out. In solid electrolyte 2 a according tothe invention of the present application, in order to enablelow-temperature operation, a uniform structure in which theabove-described respective components have been uniformly dissolved in adispersed manner needs to be formed. Therefore, a third grinding step ofgrinding the compact having undergone the above-described second heattreatment step is carried out. Furthermore, by repeatedly carrying outthe above-described compression molding step, the above-described secondheat treatment step and the above-described third grinding step in thisorder according to necessity, a material in which the respectivecomponents have been more uniformly dissolved in a dispersed manner canbe formed. Whether the above-described respective component powders havebeen uniformly dissolved in a dispersed manner can be confirmeddepending on whether component peak positions of a graph obtained by anX-ray diffractometer (XRD) are consisting of peaks derived from BZY.

Having terminated the above-described third grinding step, a secondcompression molding step of compression molding the ground material inwhich the respective components have been uniformly dissolved in adispersed manner is carried out. The second compression molding stepaccording to the present embodiment is to mold the above-describedground material into the form of above-described solid electrolyte layer2, and a disc-like compact having a thickness of 100 μm to 500 μm isformed by press molding.

As shown in FIG. 5, the above-described compact is sintered by carryingout a sintering step of heat treating at a temperature of 1400° C. to1600° C. for at least 20 hours (T₁) in an oxygen atmosphere, therebyobtaining a disc-like sintered compact constituting solid electrolytelayer 2 of the fuel cell.

As shown in FIG. 3, the above-described solid electrolyte manufacturedthrough the above-described steps shows specific changes in latticeconstant in the temperature range of 200° C. to 400° C., as plotted withthe symbol “x”. Resulting from the specific changes in lattice constant,the coefficient of thermal expansion also changes. Therefore, whenlaminating and sintering electrode layers 3 and 4 on solid electrolytelayer 2 which is the solid electrolyte sintered compact manufacturedthrough the above-described steps, large shearing stress is producedbetween solid electrolyte layer 2 and electrode layers 3, 4 because ofthe above-described changes in coefficient of thermal expansion, raisingproblems in that cracks occur in solid electrolyte layer 2 and in thatelectrode layers 3 and 4 are detached from solid electrolyte layer 2.

In the present embodiment, the third heat treatment step is carried outin order to solve the above-described problems. The above-describedthird heat treatment step can be carried out by holding disc-like solidelectrolyte layer 2 which is the above-described solid electrolytesintered compact molded by sintering at a temperature of 400° C. to1000° C. for 5 hours to 30 hours (T₂), as shown in FIG. 5.

FIG. 3 is a graph showing changes in lattice constant of a solidelectrolyte formed with 20 mol % of yttrium doped thereto, with respectto temperature changes. As plotted with the symbol “o” in FIG. 3, thelattice constant does not specifically change in a temperature rangearound 400° C. by carrying out the above-described third heat treatmentstep, so that the rate of increase in lattice constant at 100° C. to1000° C. with respect to temperature changes can be made substantiallyconstant.

Through electron microscopic observation, the mean diameter of crystalgrains in the solid electrolyte having undergone the above-describedthird heat treatment step was 1 μm. Since crystal grains of theabove-described size are obtained, high proton conductivity can beensured without increase in grain boundary surface density. In thepresent embodiment, proton conductivity at 400° C. to 800° C. was 1mS/cm to 60 mS/cm.

The lattice constant of above-described solid electrolyte 2 a at roomtemperature was 4.223 Å. Because of having the above-described latticeconstant, an appropriate doped amount of yttrium as well as absence ofspecific changes in lattice constant and coefficient of thermalexpansion around 400° C. can be confirmed.

It is noted that, in the embodiment shown in FIG. 3, the rate ofincrease in lattice constant of the above-described solid electrolyte at100° C. to 1000° C. with respect to temperature changes is approximately3.8×10⁻⁵ Å/° C., but can be set at a range of 3.3×10⁻⁵ Å/° C. to4.3×10⁻⁵ Å/° C. Accordingly, the average coefficient of thermalexpansion at 100° C. to 1000° C. can be set at 5×10⁻⁶(1/K) to9.8×10⁻⁶(1/K).

It is noted that the above-described third heat treatment step can becarried out separately from the above-described sintering step as shownin FIG. 5. Alternatively, as shown in FIG. 6, the above-describedsintering step and the above-described third heat treatment step can becarried out sequentially.

Anode electrode layer 3 is formed on one side of the above-describeddisc-like solid electrolyte having undergone the third heat treatmentstep, and cathode electrode layer 4 is formed on the other side thereof.

In the present embodiment, Ni—BZY (nickel-yttrium doped bariumzirconate) is employed as the anode electrode material constitutinganode electrode layer 3. The amount of Ni blended in Ni—BZY can be setat 67 mol % to 92 mol % (in the case of mixing NiO and BZY, the amountof NiO blended can be set at 30 wt % to 70 wt %). It is noted that, forthe above-described BZY, powder of the above-described solid electrolyteaccording to the present embodiment having undergone the third heattreatment is preferably employed. The anode electrode materiallaminating step can be carried out by grinding and mixing powder made ofNiO and BZY with a ball mill and then dissolving it in a solvent to forma paste, and applying the paste to the other side of the above-describedsolid electrolyte sintered compact by screen printing or the like.

On the other hand, an electrode material made of LSC is employed as thecathode electrode material. As the above-described LSC, a commercialproduct represented as La_(0.6)Sr_(0.4)CoO_(x) can be employed. Thecathode electrode material laminating step can be carried out bydissolving the above-described cathode electrode material in the form ofpowder into a solvent to obtain a paste and applying the paste to theone side of the above-described solid electrolyte sintered compact byscreen printing or the like.

By laminating the above-described electrode materials in predeterminedthickness respectively on the front and rear of the disc-like solidelectrolyte formed by the above-described manufacturing method, andheating them to a predetermined temperature for sintering, a solidelectrolyte laminate can be formed. For example, the above-describedmaterial constituting anode electrode layer 3 can be laminated in 50 μm,and the above-described material constituting cathode electrode layer 4can be laminated in 50 μm. Thereafter, by heating to the sinteringtemperature of the materials constituting the above-described electrodelayers and holding for a predetermined time, solid electrolyte laminate1 with anode electrode layer 3 and cathode electrode layer 4 formed onthe both sides of above-described solid electrolyte layer 2 can beformed. It is noted that the electrode material sintering step ofsintering above-described anode electrode layer 3 and the electrodematerial sintering step of sintering cathode electrode layer 4 can becarried out simultaneously or can be carried out separately.

The temperature required for sintering above-described electrode layers3 and 4 is approximately 1000° C. In the present embodiment, sinceabove-described solid electrolyte layer 2 has undergone the third heattreatment, the rate of increase in lattice constant with respect totemperature changes is constant in the temperature range of 100° C. to1000° C. The coefficient of thermal expansion is also constant incorrespondence to the lattice constant. Therefore, when formingelectrode layers 3 and 4 by sintering, large shearing stress or strainresulting from the above-described difference in coefficient of thermalexpansion will not occur at the interfaces of solid electrolyte layer 2with electrode layers 3 and 4. Therefore, a solid electrolyte laminatecan be formed without occurrence of cracks in the solid electrolytelayer and the electrode layers or detachment of the electrode layers.Since internal stress and the like are also prevented from occurring, asolid electrolyte laminate having high durability can be formed.

As described above, above-described solid electrolyte layer 2 aaccording to the present embodiment has a proton conductivity of 1 mS/cmto 60 mS/cm at 400° C. to 800° C. Therefore, even when a fuel cellincluding the above-described solid electrolyte laminate is used at atemperature less than or equal to 600° C., sufficient power generationcapacity can be ensured. Moreover, since large internal stress andinternal strain do not occur between the solid electrolyte layer and theelectrode layers, the solid electrolyte laminate has high durability,and it is possible to constitute a fuel cell having sufficientperformance at low operating temperatures.

Furthermore, in the present embodiment, LSC is employed as cathodeelectrode layer 4.

FIG. 7 shows performance of solid electrolyte laminate 1 according tothe invention of the present application configured by providing cathodeelectrode layer 4 made of LSC (La_(0.6)Sr_(0.4)CoO_(x)) for a solidelectrolyte layer composed of BZY (BaZr_(0.8)Y_(0.2)O_(3−δ)) andperformance of comparative examples each obtained by providing a cathodeelectrode layer made of another material for the above-described solidelectrolyte layer composed of BZY. It is noted that FIG. 7 showsevaluations of power generation capacity at 600° C. with each solidelectrolyte laminate mounted on a fuel cell power generation evaluatingdevice, H₂ acting on the anode electrode at a water vapor partialpressure of 0.05 atm, O₂ acting on the cathode electrode at a watervapor partial pressure of 0.05 atm, and a gas flow rate being 200ml/min.

As shown in FIG. 7, it is understood that the laminate produced usingLSC is superior to BSCF (barium strontium cobalt iron oxide), LSM andLSCF having been conventionally used in performance of any of voltage,current density and surface power density. Therefore, it is possible tomanufacture a fuel cell operating in an intermediate temperature rangeless than or equal to 600° C. and having high performance through use ofthe solid electrolyte laminate constructed as described above.

In the above-described embodiment, the disc-like sintered compactconstituting solid electrolyte layer 2 is formed first, and thenelectrode layers 3 and 4 are laminated on this disc-like sinteredcompact serving as a support member, but the manufacturing method is notlimited to the above-described method.

For example, a technique for first forming an anode electrode layer 23shown in FIG. 8 and then successively laminating solid electrolyte layer22 and cathode electrode layer 24 on this anode electrode layer 23serving as a support member can be employed. FIG. 10 shows a flow chartof a manufacturing process of a solid electrolyte laminate formed bythis technique.

An anode electrode compact to be anode electrode layer 23 can be formedby an anode electrode material preparing step of mixing and grindingBaCO₃, ZrO₂, Y₂O₃, and Ni and an anode electrode molding step ofcompression molding the above-described anode electrode material to forman anode electrode compact to be the anode electrode layer. In thismanufacturing method, since the anode electrode compact (anode electrodelayer 23) serves as a support member for solid electrolyte layer 22 andcathode electrode layer 24, the thickness of the anode electrode compactis set large. For example, it is preferably set at approximately 500 μmto 1 mm.

The technique for laminating the solid electrolyte layer on theabove-described anode electrode compact can be carried out as follows.That is, the above-described first grinding step, the above-describedfirst heat treatment step, the above-described second grinding step, theabove-described first compression molding step, the above-describedsecond heat treatment step, and the above-described third grinding stepare carried out to form a ground product of BZY, similarly to theabove-described manufacturing method.

Next, a paste forming step of forming the above-described ground productinto paste and a solid electrolyte laminating step of laminating theabove-described ground product formed into paste on one side of theabove-described anode electrode compact are carried out. Sinceabove-described solid electrolyte layer 22 does not serve as a supportmember in this embodiment, its thickness can be set as small as 10 μto100 μm. The above-described solid electrolyte laminating step can becarried out by screen printing or the like.

Then, an anode electrode-solid electrolyte sintering step of heattreating the laminate molded in the above-described solid electrolytelaminating step at a temperature of 1400° C. to 1600° C. for at least 20hours in an oxygen atmosphere and a third heat treatment step of holdingthe laminate having undergone the above-described anode electrode-solidelectrolyte sintering step for a predetermined time at a temperaturelower than in the above-described anode electrode-solid electrolytesintering step are carried out. Similarly to the first embodiment, theabove-described anode electrode-solid electrolyte sintering step can becarried out by heat treatment at a temperature of 1400° C. to 1600° C.for at least 20 hours in an oxygen atmosphere. The above-described thirdheat treatment step can also be carried out by holding at a temperatureof 400° C. to 1000° C. for 5 hours to 30 hours (T₂), similarly to thefirst embodiment.

A cathode electrode material laminating step of laminating a cathodeelectrode material made of lanthanum strontium cobalt oxide (LSC) on oneside of a thin-film solid electrolyte having undergone theabove-described third heat treatment step and a cathode electrodesintering step of heating to or above the sintering temperature of theabove-described cathode electrode material are carried out. Theabove-described cathode electrode sintering step can be carried outsimilarly to the above-described embodiment. Above-described solidelectrolyte laminate 21 can also be formed through these steps.

The scope of the invention of the present application is not limited tothe above-described embodiments. It should be understood that theembodiments disclosed herein are illustrative and non-restrictive inevery respect. The scope of the present invention is defined by theclaims not by the meaning above, and is intended to include anymodification within the meaning and scope equivalent to the terms of theclaims.

INDUSTRIAL APPLICABILITY

A solid electrolyte laminate that includes a solid electrolyte made ofyttrium-doped doped barium zirconate (BZY) having excellentsinterability and proton conductivity as well as a cathode electrodelayer made of lanthanum strontium cobalt oxide (LSC) well compatiblewith this and that can exert high performance at a temperature less thanor equal to 600° C. can be provided.

REFERENCE SIGNS LIST

1 solid electrolyte laminate; 2, 22 solid electrolyte layer; 2 a solidelectrolyte; 3, 23 anode electrode layer; 4, 24 cathode electrode layer.

1. A solid electrolyte laminate comprising: a solid electrolyte layerhaving proton conductivity; and a cathode electrode layer laminated onone side of said solid electrolyte layer and made of lanthanum strontiumcobalt oxide (LSC).
 2. The solid electrolyte laminate according to claim1, wherein said solid electrolyte layer is formed from yttrium-dopedbarium zirconate (BZY), and a doped amount of yttrium is 15 mol % to 20mol %.
 3. The solid electrolyte laminate according to claim 2, whereinthe rate of increase in lattice constant of said solid electrolyte layerat 100° C. to 1000° C. with respect to temperature changes is 3.3×10⁻⁵Å/° C. to 4.3×10⁻⁵ Å/° C.
 4. The solid electrolyte laminate according toclaim 2, wherein said yttrium-doped barium zirconate (BZY) is apolycrystalline substance containing a plurality of crystal grains, anda mean diameter of said crystal grains is more than or equal to 1 μm. 5.The solid electrolyte laminate according to claim 2, wherein the latticeconstant of said solid electrolyte layer at room temperature is 4.218 Åto 4.223 <.
 6. The solid electrolyte laminate according to claim 2,wherein proton conductivity of said solid electrolyte layer at 400° C.to 800° C. is 1 mS/cm to 60 mS/cm.
 7. The solid electrolyte laminateaccording to claim 1, further comprising an anode electrode layerlaminated on the other side of said solid electrolyte layer and made ofnickel-yttrium doped barium zirconate (Ni—BZY).
 8. A method formanufacturing the solid electrolyte laminate as defined in claim 1,comprising: a first grinding step of mixing and grinding BaCO₃, ZrO₂ andY₂O₃ to obtain a first mixture; a first heat treatment step of heattreating said first mixture; a second grinding step of grinding thefirst mixture having undergone said first heat treatment step again toobtain a second mixture; a first compression molding step of compressionmolding said second mixture to obtain a first compact; a second heattreatment step of heat treating said first compact; a third grindingstep of grinding the first compact having undergone said second heattreatment step to obtain a ground product; a second compression moldingstep of compression molding said ground product to obtain a secondcompact; a solid electrolyte sintering step of heat treating said secondcompact at a temperature of 1400° C. to 1600° C. for at least 20 hoursin an oxygen atmosphere to obtain a sintered compact; a third heattreatment step of holding said sintered compact at a temperature lowerthan in said solid electrolyte sintering step; a cathode electrodematerial laminating step of laminating an electrode material made ofsaid lanthanum strontium cobalt oxide (LSC) on one side of the sinteredcompact having undergone said third heat treatment step; an anodeelectrode material laminating step of laminating an electrode materialmade of nickel-yttrium doped barium zirconate (Ni—BZY) on the other sideof the sintered compact having undergone said third heat treatment step;and an electrode material sintering step of heating the sintered compactwith said electrode materials laminated thereon to or above a sinteringtemperature of said electrode materials.
 9. A method for manufacturingthe solid electrolyte laminate as defined in claim 1, comprising: ananode electrode material preparing step of mixing BaCO₃, ZrO₂, Y₂O₃, andNi; an anode electrode molding step of compression molding an anodeelectrode material to form an anode electrode compact to be an anodeelectrode layer; a first grinding step of mixing and grinding BaCO₃,ZrO₂ and Y₂O₃ to obtain a first mixture; a first heat treatment step ofheat treating said first mixture; a second grinding step of grinding thefirst mixture having undergone said first heat treatment step again toobtain a second mixture; a first compression molding step of compressionmolding said second mixture to obtain a compact; a second heat treatmentstep of heat treating said compact; a third grinding step of grindingthe compact having undergone said second heat treatment step to obtain aground product; a paste forming step of forming said ground product intopaste to obtain paste; a solid electrolyte laminating step of laminatingsaid paste on one side of said anode electrode compact to obtain a firstlaminate including a thin-film solid electrolyte layer; an anodeelectrode-solid electrolyte sintering step of heat treating said firstlaminate at a temperature of 1400° C. to 1600° C. for at least 20 hoursin an oxygen atmosphere; a third heat treatment step of holding thefirst laminate having undergone said anode electrode-solid electrolytesintering step at a temperature lower than in said anode electrode-solidelectrolyte sintering step; a cathode electrode material laminating stepof laminating a cathode electrode material made of said lanthanumstrontium cobalt oxide (LSC) on one side of the solid electrolyte layerincluded in the first laminate having undergone said third heattreatment step to obtain a second laminate; and a cathode electrodesintering step of heating said second laminate to or above a sinteringtemperature of said cathode electrode material.
 10. The method formanufacturing the solid electrolyte laminate according to claim 8,wherein said third heat treatment step is carried out by holding at atemperature of 400° C. to 1000° C. for 5 hours to 30 hours.
 11. Themethod for manufacturing the solid electrolyte laminate according toclaim 9, wherein said third heat treatment step is carried out aftercooling the first laminate having undergone said anode electrode-solidelectrolyte sintering step to ordinary temperature.
 12. The method formanufacturing the solid electrolyte laminate according to claim 9,wherein said anode electrode-solid electrolyte sintering step and saidthird heat treatment step are carried out sequentially.
 13. A fuel cellcomprising the solid electrolyte laminate as defined in claim 1.